(1) Field of the Invention
The present invention relates to directional control valve (hydraulic pressure changeover valve) used to control the interlocking, stop, acceleration, deceleration of an actuator and the acting direction thereof in hydraulic circuit.
(2) Description of the Prior Art
In usual, spool-type changeover valves have been used as a directional control valve in hydraulic circuit. However, there have several defects as hereinafter described.
First, a problem of internal leakage will be explained. In spool-type changeover valves a gap is provided between the spool land and the valve body to help the smooth sliding motion in the spool. Properties of the gap are determined usually under consideration of strain of the valve body caused by hydraulic pressure or tightening strength and variation of the gap length dependent on the temperature difference between the spool and the valve body at hot state also consideration of accuracy of machining. The sealing function in the sliding portion therefore has limitations and no effective means is found to prevent the leakage completely.
Next, a problem of shock produced during the changeover will be explained. In spool-type changeover valves, a defect of the construction i.e., large rate of variation of the path cross-section in comparison to action of the spool, causes very strong shock during the changeover, resulting in the apparatus damage.
In order to prevent the leakage, pilot check valves are arranged in required portions to help the function. In order to eliminate the shock also, pilot choke valves are used to decelerate the spool action and retard the changeover. However, these are not essential solutions of the problems.
A more troublesome problem is seizure of the spool. When oil including dust flows through the narrow gap between the spool land and the valve body on account of the internal leakage, the dust may fill the gap and pressure unbalance in the vertical direction transfer the spool land in one direction to obstruct the spool action. Balance grooves may be arranged on the spool land to eliminate the seizure, however, this remedy is of little effect.
Spool-type changeover valves have been used in spite of above mentioned problems. Accordingly, it is necessary to develop directional control valves with more safe, secure and economic properties.